Wavelength locker integrated with a silicon photonics system

ABSTRACT

A wavelength locker integrated with a silicon photonics transmission system comprising a silicon-on-insulator (SOI) substrate and an input via a power tap coupler to receive a fraction of a transmission signal with one or more frequencies from a primary output path of the silicon photonics transmission system. The wavelength locker further includes a splitter configured to split the input to a first signal in a first path and a second signal in a second path and a first delay-line-interferometer (DLI) coupled to the second path to receive the second signal and configured to generate an interference spectrum and output at least two sub-spectrums tunable to keep quadrature points of the sub-spectrums at respective one or more target frequencies. The wavelength locker is configured to generate an error signal fed back to the silicon photonics transmission system for locking the one or more frequencies at the one or more target frequencies.

CROSS-REFERENCE TO RELATED APPLICATIONS

N/A

BACKGROUND OF THE INVENTION

The present invention relates to optical telecommunication techniques.More particularly, the present invention provides a wavelength lockerintegrated in silicon photonics transmission system.

Over the last few decades, the use of communication networks exploded.In the early days Internet, popular applications were limited to emails,bulletin board, and mostly informational and text-based web pagesurfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. For example, a social network like Facebook processes more than500 TB of data daily. With such high demands on data and data transfer,existing data communication systems need to be improved to address theseneeds.

Progress in computer technology (and the continuation of Moore's Law) isbecoming increasingly dependent on faster data transfer between andwithin microchips. Optical interconnects may provide a way forward, andsilicon photonics may prove particularly useful, once integrated on thestandard silicon chips. 40-Gbit/s and then 100-Gbit/s data rates WDMoptical transmission over existing single-mode fiber is a target for thenext generation of fiber-optic communication networks. The big hangup sofar has been the fiber impairments like chromatic dispersion that areslowing the communication signal down. Everything is okay up to 10Gbits/s plus a little, but beyond that, distortion and attenuation taketheir toll. Many approaches are proposed on modulation methods fortransmitting two or more bits per symbol so that higher communicationrates can be achieved. Beyond the light modulation for datatransmission, the MUX/DEMUX of light signals is another one of essentialbuilding blocks for the optical communication network. All these networkbuilding blocks integrated on silicon chips as silicon photonic deviceshave many advantages over conventional stand-alone optical andelectrical devices.

In particular, silicon photonic devices have been applied indense-wavelength-division multiplexing (DWDM) optical transmissionnetworks, in which DEMUX/MUX of light signals require precise wavelengthtargeting and control over environment temperature change. Therefore,improved silicon photonics wavelength locking techniques and devices aredesired.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to optical telecommunication techniques.More particularly, the present invention provides a wavelength lockerintegrated in silicon photonics transmission system. Merely by examples,the present invention discloses several configurations of wavelengthlocker based on delay-line-interferometer (DLI) that is integrated witha silicon photonic transmission system-on-chip on a silicon-on-insulatorsubstrate for high data rate DWDM optical communications, though otherapplications are possible.

In modern electrical interconnect systems, high-speed serial links havereplaced parallel data buses, and serial link speed is rapidlyincreasing due to the evolution of CMOS technology. Internet bandwidthdoubles almost every two years following Moore's Law. But Moore's Law iscoming to an end in the next decade. Standard CMOS silicon transistorswill stop scaling around 5 nm. And the internet bandwidth increasing dueto process scaling will plateau. But Internet and mobile applicationscontinuously demand a huge amount of bandwidth for transferring photo,video, music, and other multimedia files. This disclosure describestechniques and methods to improve the communication bandwidth beyondMoore's law.

In an embodiment, the present invention provides a wavelength lockerintegrated with a silicon photonics transmission system. The wavelengthlocker includes a silicon-on-insulator (SOI) substrate on which thesilicon photonics transmission system is formed. Further, the wavelengthlocker includes a power tap coupler to receive a fraction of atransmission signal from the silicon photonics transmission system as aninput signal with one or more frequencies. Additionally, the wavelengthlocker includes a splitter configured to split the input signal to afirst signal in a first path and a second signal in a second path. Thewavelength locker further includes a first delay-line-interferometer(DLI) coupled to the second path to receive the second signal andconfigured to generate an interference spectrum and output a firstsub-spectrum and a second sub-spectrum shifted in frequency domain byhalf of a free-spectral-range to form a plurality of quadrature pointsat respective one or more target frequencies. Furthermore, thewavelength locker includes a first detector coupled to the first pathfor detecting a reference power of the first signal and a seconddetector and a third detector for respectively detecting a first powerof the first sub-spectrum and a second power of the second sub-spectrumdepended on frequency to generate an error signal fed back to thesilicon photonics transmission system for locking the one or morefrequencies at corresponding one or more target frequencies.

In an alternative embodiment, the present invention provides a siliconphotonics transmission system including one or more DFB lasers to outputa transmission signal to an output port, a power tap coupler coupled tothe output port to provide a primary output path for outputting thetransmission signal and a secondary path for providing a fraction of thetransmission signal as an input signal of a wavelength locker forlocking corresponding laser frequency of each of the one or more DFBlasers to a respective target frequency. The wavelength locker includesa splitter configured to split the input to a first signal in a firstpath and a second signal in a second path. Furthermore, the wavelengthlocker includes a first delay-line-interferometer (DLI) coupled to thesecond path to receive the second signal and configured to generate aninterference spectrum and to output at least two sub-spectrums having arelative phase shift tunable to keep a plurality of quadrature points ofthe two sub-spectrums at respective target frequencies selected from aset of channels. Moreover, the wavelength locker includes a firstdetector coupled to the first path for detecting the first signal as areference and one or more second detectors for detecting powers of theat least two sub-spectrums varying with frequency to generate adifferential error signal fed back to the silicon photonics transmissionsystem for locking the one or more frequencies respectively atcorresponding target frequencies selected from the set of channels.Optionally, the set of channels is DWDM channels of ITU grid.Optionally, the set of channels has a channel spacing of 100 GHz, or 50GHz, or 25 GHz. Optionally, the set of channels is a set of gridlesschannels.

The present invention achieves these benefits and others in the contextof known silicon waveguide laser communication technology. However, afurther understanding of the nature and advantages of the presentinvention may be realized by reference to the latter portions of thespecification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1 is a simplified diagram of a wavelength locker integrated in asilicon photonics system-on-chip according to some embodiments of thepresent invention.

FIG. 2 is a simplified diagram of the wavelength locker of FIG. 1including one delay-line-interferometer (DLI) outputting twophase-shifted interference sub-spectrums according to an embodiment ofthe present invention.

FIG. 3 is a simplified diagram of the wavelength locker of FIG. 1including one DLI outputting two phase-shifted interferencesub-spectrums according to another embodiment of the present invention.

FIG. 4 is a plot of two normalized PD currents representing twophase-shifted interference sub-spectrums of the DLI in frequency domainaccording to an embodiment of the present invention.

FIG. 5 is a simplified diagram of the wavelength locker including twoDLIs totally outputting four phase-shifted interference sub-spectrumsaccording to an embodiment of the present invention.

FIG. 6 is a simplified diagram of the wavelength locker including oneDLI and a 2×4 MIMI coupler outputting four phase-shifted interferencesub-spectrums according to another embodiment of the present invention.

FIG. 7 is a plot of four normalized PD currents representing fourphase-shifted interference sub-spectrums of the DLI in frequency domainaccording to an embodiment of the present invention.

FIG. 8 is formula of an error signal generated from two PD currentsmeasured from two interference sub-spectrums by the wavelength locker.

FIG. 9 is a simplified diagram of the wavelength locker of FIG. 2 with acalibration port according to an embodiment of the present invention.

FIG. 10 is a simplified diagram of the wavelength locker of FIG. 2 witha calibration port according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to optical telecommunication techniques.More particularly, the present invention provides a wavelength lockerintegrated in silicon photonics transmission system. Merely by examples,the present invention discloses several configurations of wavelengthlocker based on delay-line-interferometer (DLI) that is integrated witha silicon photonic transmission system-on-chip on a silicon-on-insulator(SOI) substrate for high data rate DWDM optical communications, thoughother applications are possible.

In the last decades, with advent of cloud computing and data center, theneeds for network servers have evolved. For example, the three-levelconfiguration that have been used for a long time is no longer adequateor suitable, as distributed applications require flatter networkarchitectures, where server virtualization that allows servers tooperate in parallel. For example, multiple servers can be used togetherto perform a requested task. For multiple servers to work in parallel,it is often imperative for them to be share large amount of informationamong themselves quickly, as opposed to having data going back forththrough multiple layers of network architecture (e.g., network switches,etc.).

Leaf-spine type of network architecture is provided to better allowservers to work in parallel and move data quickly among servers,offering high bandwidth and low latencies. Typically, a leaf-spinenetwork architecture uses a top-of-rack switch that can directly accessinto server nodes and links back to a set of non-blocking spine switchesthat have enough bandwidth to allow for clusters of servers to be linkedto one another and share large amount of data.

In a typical leaf-spine network today, gigabits of data are shared amongservers. In certain network architectures, network servers on the samelevel have certain peer links for data sharing. Unfortunately, thebandwidth for this type of set up is often inadequate. It is to beappreciated that embodiments of the present invention utilizes PAM(e.g., PAM4, PAM8, PAM12, PAM16, etc.) in leaf-spine architecture thatallows large amount (up terabytes of data at the spine level) of data tobe transferred via optical network.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

FIG. 1 is a simplified diagram of a wavelength locker integrated with asilicon photonics system-on-chip according to some embodiments of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Asshown schematically in FIG. 1, the wavelength locker 100 is integratedwith a silicon-photonics system-on-chip 10 formed on asilicon-on-insulator (SOI) substrate 01. The silicon-photonicssystem-on-chip 10 is a transmission system having an output port 101. Apower tap coupler 110 is a coupled to the output port 101 to provide aprimary output path 102 for outputting a transmission signal and asecondary tap path 111 taking a fraction of the transmission signal asan input signal 111 of the wavelength locker 100. Optionally, the powertap coupler includes a cross tap port 112 configured to receive a tappedsignal travelling in reverse direction from the primary output path 102.Optionally, the power tap coupler 110 is a waveguide device formed onthe same SOI substrate 01. Optionally, the transmission signal can bemultiplexed from one or more frequencies generated respectively by oneor more DFB lasers coupled to the silicon photonics transmission system10.

In an embodiment, the wavelength locker 100 includes a splitter 120configured to split the input signal 111 into two paths with two signalsin substantially equal power. A first signal carries substantially halfpower of the input signal with the one or more frequencies into a firstpath 121 which is terminated by a detector PD0 140 and a second signalcarries substantially half power of the input signal with the one ormore frequencies into a second path 122 into a delay-line interferometer(DLI) 130. The DLI 130 is configured to process the second signal togenerate an interference spectrum and output at least two sub-spectrumswith a phase shift associated with a free-spectral range of the DLI 130respectively to at least two signal paths 132. More detectors PDx 141are used to detect powers in the at least two signal paths 132respectively, based on which a feedback signal can be sent to thesilicon photonics transmission system 10 for adjusting the correspondingone or more laser frequencies back to target frequencies.

FIG. 2 is a simplified diagram of the wavelength locker of FIG. 1including one delay-line-interferometer (DLI) outputting twophase-shifted interference sub-spectrums according to an embodiment ofthe present invention. This diagram is merely an example, which shouldnot unduly limit the scope of the claims. One of ordinary skill in theart would recognize many variations, alternatives, and modifications. Asa specific embodiment, the wavelength locker 200 includes a splitter 220configured as a 1×2 power splitter to receive an input signal 111, afraction of the transmission signal, and split into a first signal in afirst path 221 and a second signal in a second path 222 with substantialequal intensity. Both the first signal and the second signal carry theone or more frequencies in the original transmission signal directlyfrom the one or more DFB lasers, which are the subject frequencies to belocked by the wavelength locker 200 according to the present disclosure.The first path 221 is terminated by a photodiode PD0 240 for convertinglight intensity of the first signal to a PD current, serving as areference. Additionally, the second signal is guided to a MMI coupler2301 of a waveguide-based DLI 230.

The DLI 230 is a Mach-Zehnder interferometer having two waveguide arms:a first arm 2311 and a second waveguide arm 2312, to receive twoequal-intensity waves of the second signal respectively. The first arm2311 is selected to be longer in length than the second arm 2312 tocreate a phase delay in the first arm 2311 due to the length difference.The phase delay induces an interference spectrum as the two waves arerecombined after respectively traveling through the two waveguide arms.The interference spectrum is characterized by a free-spectral rangeFSR=c/(n_(g)ΔL) represented by a spacing between two neighboring peaksalong the frequency axis, where n_(g) is group index of the waveguidematerial and ΔL is the length difference. Optionally, the two recombinedwaves interfere each other constructively or destructively after theyare recombined at another 2×2 MMI coupler 2302, converting a phase-keyedsignal into an amplitude-keyed signal. The 2×2 MMI coupler 2302 isconfigured to output a first sub-spectrum representing half-poweredinterference spectrum into a first signal path and a second sub-spectrumrepresenting same half-powered interference spectrum but with a shiftedphase of π in frequency domain (see FIG. 4).

For the case that the transmission signal carrying one or morefrequencies that are supposed to be locked at the one or more targetfrequencies, the DLI 230 can be configured, by tuning its phase delay,to set the FSR to be equal to 2× (or twice) of the difference betweenthe two neighboring target frequencies of a set of channels in certaingrid with a fixed channel spacing. Choosing the FSR to be 2× the channelspacing allows the wavelength locker 200 to be kept in a smallest size,reducing chip area needed for manufacturing the silicon waveguide-basedwavelength locker. Optionally, each target frequency can be chosen fromone of a plurality of quadrature points of the two interferencesub-spectrums having a relative phase shift of π crossing each other. Ateach quadrature point intensity of the first sub-spectrum (solid curvein FIG. 4) varies with frequency in a negative slope and intensity ofthe second sub-spectrum (dotted curve in FIG. 4) varies with frequencyin a positive slope. Whenever one of the one or more frequencies of thetransmission signal is off the target frequency, the two intensities ofthe first and the second sub-spectrum will change in opposite direction,resulting a simply sign flip of a difference between the twointensities. Conversely, the quadrature point frequency can beconveniently used as target frequency at which the correspondingtransmission signal frequency is aimed to lock by the wavelength locker.Additionally, two nearest quadrature points that are used as targetfrequencies can be just two nearest channels for transmission. Forexample, the channels for transmission are DWDM channels of ITU grid.The difference between the two quadrature-point frequencies is just thechannel spacing of the ITU grid, such as 100 GHz, 50 GHz, or 25 GHz.

Referring to FIG. 2, for example, the input signal 111 tapped from theprimary output path carries two frequencies at f1 and f2 generated fromtwo lasers of the silicon photonics transmission system 10. After theinput signal 111 being split to two halves, the first signal 221 and thesecond signal 222, the second signal 222 is further split by the 1×2 MMIcoupler 2301 into two waves respectively passed into two waveguide armsof the DLI 230. The 2×2 MMI coupler 2302 recombines the two waves togenerate the interference spectrum and outputs the first sub-spectrum toa first signal path 231 and the second sub-spectrum to a second signalpath 232 having a phase shift of 180° degrees. Each signal path isindependently terminated by a photodiode: PD1 241 for measuring signalintensity of the first sub-spectrum in the first signal path 231 interms of a first PD current and PD2 242 for measuring signal intensityof the second sub-spectrum in the second signal path 232 in terms of asecond PD current. In an embodiment, either the first PD current or thesecond PD current is normalized by the reference current obtained by PD0240 (by measuring the signal intensity in the first path 221) toeliminate the possible power fluctuation entering the two waveguide armsof the DLI 230. As shown in FIG. 4, the two normalized PD currentsrepresenting two phase-shifted interference sub-spectrum waves of theDLI are plotted in frequency domain. Both interference sub-spectrums aresine waves interleaved to each other with same normalized amplitude anda same period equal to the FSR of the DLI 230 and a plurality ofquadrature points equally distributed along the frequency axis as thetwo interference sub-spectrums crosses each other. A quadrature point isan intersection of the two sine waves along a frequency axisrespectively at two target frequencies f1′ and f2′, e.g., two DWDMnarrow-band channel frequencies in ITU grid. By design, two DFB lasersassociated with the silicon photonics transmission system 10 generateslight at two frequencies f1 and f2 at the two DWDM channels multiplexedinto a single transmission signal. In other words, the wavelength locker200 is provided to tune the two laser frequencies f1 and f2 to the twotarget channel frequencies, i.e., to lock f1=f1′ and f2=f2′. Optionally,the target frequencies can be two or more gridless channels having achannel spacing.

The locking of the laser frequency can be achieved using an error signalgenerated by the first PD current and the second PD current as afeedback signal for the corresponding laser diode. FIG. 8 is a formulaof the error signal generated from two PD currents measured from the twointerference sub-spectrums outputted by the 2×2 MMI coupler of thewavelength locker. Referring back to FIG. 2, the first PD current in thefirst signal path 231 represents the intensity W1 _(bar) of the firstsub-spectrum and the second PD current in the second signal path 232represents the intensity W1 _(cross) of the second sub-spectrum.Ideally, referring to FIG. 8, both W1 _(bar) and W1 _(cross) should beequal to keep the error signal Err=0. As a differential signal, the signof the error signal Err can also indicate which direction the targetfrequency is shifted off from the quadrature point. The error signal Erris fed back to the laser diode for tuning the laser frequencyaccordingly to lock the laser frequency (e.g. f1) back to the targetfrequency (e.g., f1′). For example, laser 1 frequency f1 is off bydrifting to a lower frequency than the target frequency f1′. Then, theerror signal Err records a positive value and suggests the laserfrequency should be tuned in positive direction (increase from a presentvalue f1) towards the target frequency f1′. In principle, any number oftarget frequencies can be assigned to respect quadrature points of theDLI interference spectrum that can be used for locking the any number ofcorresponding laser frequencies thereof.

Optionally, in order to control the quadrature points to besubstantially fixed to the one or more target frequencies and toovercome possible temperature drift of these quadrature points, the DLI230 also includes a resistive heater 2315, as shown in FIG. 2, disposedaround the first arm 2311 for tuning the group index value to compensatethe SFR drift due to environmental temperature change.

Optionally, the wavelength locker 200 includes a temperature sensor 250disposed on the SOI substrate 01 for monitoring local temperature nearthe DLI 230 for guiding a tuning of the two-beam interference spectrumand calibrating the DLI 230 against thermal drift. Thus, the DLI 230 canbe tuned and held more accurately at frequency grid to ensure the laserfrequency be locked to the target frequency. The temperature sensor 250can be used to calibrate the small but non-zero thermal drift of the DLI230 over temperature.

FIG. 3 is a simplified diagram of the wavelength locker of FIG. 1including one DLI outputting two phase-shifted interferencesub-spectrums according to another embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. In some embodiments,due to device variation or other unknown factors, power splitting isuneven for the first path 321 and the second path 322 after the 1×2power splitter 320. The wavelength locker 300, being substantiallysimilar to the wavelength locker 200, further includes a variableoptical attenuator (VOA) disposed to the first path. VOA0 360 in thefirst path 321 is for adjusting the signal intensity in the first path321 before coupled to a photodiode PD0 340. The intensity adjustment byVOA is based on intensity measurement by the PD0 340 which defines thereference intensity for normalizing the processed signals by the DLI330. Similarly, in two signal paths 331 and 332, two VOAs: VOA1 361 andVOA2 362, are respectively added for balancing the intensity of the twoprocessed signals before being converted to PD currents by twophotodiodes: PD1 341 and PD2 342, based on which an error signal can beobtained for adjusting the laser frequency to lock it at a targetfrequency. Similar to the wavelength locker 200 illustrated in FIG. 2,the DLI 330 includes an MMI coupler 3301, a 2×2 MMI coupler 3302, twowaveguide arms 3311 and 3312, and a resistive heater 3315 thatcorrespond to the MMI coupler 2301, the 2×2 MMI coupler 2302, the twowaveguide arms 2311 and 2312, and the resistive heater 2315,respectively.

In some embodiments, the wavelength locker under a same operationprinciple can be extended for locking the transmission frequencies totarget frequencies with finer channel spacing. FIG. 5 is a simplifieddiagram of the wavelength locker including two DLIs with fourphase-shifted interference waves according to an embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Asshown, the wavelength locker 400 includes two waveguide-based DLIs 430Aand 430B having a 1×4 MMI coupler 4301 to receive an input signal 422carrying four laser frequencies generated by four laser diodes: Laser 1,Laser 2, Laser 3, and Laser 4, associated with a silicon photonicstransmission system 10 formed on the SOI substrate 01. The input signal422 is a second one of two paths split by a 1×2 power splitter 420 froma tapped signal 111 as a fraction of a transmission signal from aprimary output path 102 of a silicon photonics transmission system 10while the first one of the two paths carries a reference signal (equalto 50% of the input signal 422) and is terminated by a detector PD0 440and optionally including a VOA0 460 for adjusting power.

Referring to FIG. 5, a first DLI 430A includes two waveguide arms, afirst arm 4311 and a second arm 4312, from the output of the 1×4 MIMIcoupler 4301 with the first arm 4311 being set to be longer than thesecond arm 4312 to create a phase delay between the two waves of thesame frequency in the first arm and the second arm. The two waves arerecombined at a 2×2 MMI coupler 4302 to generate a first interferencespectrum. A second DLI 430B includes two waveguide arms, a third arm4313 and a fourth arm 4314, from the output of the 1×4 MMI coupler 4301with the third arm 4313 being set to be longer than the fourth arm 4314to create a phase delay between the two waves of the same frequency inthe third arm and the fourth arm. The two waves are recombined atanother 2×2 MIMI coupler 4303 to generate a second interferencespectrum. The 2×2 MIMI coupler 4302 outputs first two interferencesub-spectrums associated with the first interference spectrumrespectively into a first signal path 431 and a second signal path 432having a relative phase shift of 180 degrees. The 2×2 MIMI coupler 4303outputs second two interference sub-spectrums associated with the secondinterference spectrum respectively into a third signal path 433 and afourth signal path 434 having a relative phase shift of 180 degrees.Optionally, the second two interference sub-spectrums are configured tohave a phase shift of 90 degrees relative to the first two interferencesub-spectrums. The sub-spectrum in the third signal path 433 has a phaseshift of 90 degrees off the sub-spectrum in the first signal path 431.The sub-spectrum in the fourth signal path 434 has a phase shift of 90degrees off the sub-spectrum in the second signal path 432.

In a specific embodiment, each signal path of the two DLIs is terminatedby a photodiode, PD1 441 through PD4 444 respectively for the foursignal paths 431 through 434 for converting the signal intensity to acorresponding PD current. As a result, the intensity of eachinterference sub-spectrum can be represented by a PD current plotted infrequency domain. FIG. 7 is a plot of four normalized PD currentsrepresenting four phase-shifted interference sub-spectrums of the DLI infrequency domain according to an embodiment of the present invention. Asshown, curve 1 and curve 3 are two PD currents, with their amplitude(intensity) being normalized with a reference intensity measured by thePD0 440 in the first path 421, representing two interferingsub-spectrums outputted from the MMI coupler 4302 of the first DLI 430Ato set two target frequencies assigned to Laser 1 and Laser 3 at twoquadrature points. Curve 2 and curve 4 are other two PD currentsrepresenting two interference sub-spectrums outputted from the MIMIcoupler 4303 of the second DLI 430B to set two other target frequenciesassigned to Laser 2 and Laser 4 at two other quadrature points. The twotarget frequencies assigned for Lasers 1 and 3 can be two DWDM channelsselected from ITU grid having a channel spacing (e.g., 100 GHz spacingbetween Laser 1 and Laser 3). Curves 1 and 3 are interleaved with curves2 and 4. The two target frequencies assigned for Lasers 2 and 4 can betwo additional DWDM channels selected from ITU grid having a finerchannel spacing (e.g., 50 GHz between Laser 1 and Laser 2 and betweenLaser 3 and Laser 4). In an alternative view, the FSR is equal to 4× ofthe finer channel spacing between two neighboring target frequencieswhich has been defined earlier as two times of the period of eachinterference wave (1, or 2, or 3, or 4) in frequency domain. Optionally,the target frequencies are selected from DWDM narrow-band channels atITU grid with a channel spacing of 100 GHz, or 50 GHz, or 25 GHz, or12.5 GHz. Optionally, the target frequencies are selected from two ormore gridless channels.

Optionally, the first DLI 430A includes a resistive heater 4315 toadjust the group index associated with the waveguide material and helpto tune the phase shift and central frequency position to maintain thequadrature points at the two target frequencies assigned for Laser 1 andLaser 3. Similarly, the second DLI 430B includes a resistive heater 4316for achieving the same purpose to keep the corresponding quadraturepoints at two target frequencies assigned for Laser 2 and Laser 4.Optionally, each signal path is inserted a VOA, such as VOA1 461 forsignal path 431, VOA2 462 for signal path 432, VOA3 463 for signal path433, and VOA4 464 for signal path 434, for balancing the power in eachsignal path to eliminate possible signal fluctuation.

In an alternative embodiment, the two DLIs in the wavelength locker 400can be replaced by one DLI plus an optical hybrid coupler in a samewaveguide-based wavelength locker for similar operations with channelsin finer DWDM grid or gridless DWDM channels. FIG. 6 is a simplifieddiagram of the wavelength locker including one DLI and a 2×4 MMI coupleroutputting four phase-shifted interference sub-spectrums according toanother embodiment of the present invention. A 2×4 MMI coupler 5302 haseach of the outputs at 90° phase offset. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. As shown, the wavelength locker 500 includes a 1×2power splitter 520 to split an input 111 to two paths with equalintensity, a first path 521 terminated by a photodiode PD0 540 tomeasuring the reference intensity and a second path 522 being guided toa DLI 530 via a MMI coupler 5301 into two waveguide arms. A first arm5311 is set to be longer than the second arm 5312 to create a phasedelay for generating an interference spectrum when the two paths arerecombined at the 2×4 MMI coupler 5302. The 2×4 MMI coupler 5302 outputsfour interference sub-spectrums respectively passed to four signal paths531-534 having a relative phase shift of 90 degrees. These fourinterference sub-spectrums in frequency domain are substantially thesame as the four interference sub-spectrums out of the two pairs ofsignal paths 431-432 of a first DLI 430A and 433-434 of a second DLI430B shown in FIG. 5.

In an embodiment, the wavelength locker includes one DLI having a firstarm and a second arm with a phase delay to induce an interferencespectrum at a 2×N MMI coupler, wherein N is 2^(n) and n is naturalnumber. The 2×N MMI coupler is configured to output N sub-spectrumsrespectively to N signal paths in same power with a relative 2π/N phaseshift equal to 1/N of the free-spectral-range of the DLI. The Nsub-spectrums cross each other to yield a plurality of quadraturepoints. The corresponding frequency at each quadrature point can beselected to be one target frequency. At each quadrature point, opticalintensity of one sub-spectrum decreases with increasing frequency andoptical intensity of the next sub-spectrum increases with increasingfrequency. An error signal based on frequency differential at thequadrature yields a positive/negative sign change when the frequencycrosses this duadrature point, making it easier to be detected and usingthe sign change as a feedback for locking transmission frequency at thetarget frequency defined by the quardrature point. Optionally, thewavelength locker includes a resistive heater disposed around the firstarm of the DLI for tuning the interference spectrum in frequency domainto keep the quadrature points of the N sub-spectrums crossing each otherrespectively at one or more target frequencies.

Similar to the wavelength locker 400, the wavelength locker 500 alsouses a photodiode to measure intensity of each interference sub-spectrumin each of the four signal paths, for example, PD1 541 for signal path531, PD2 542 for signal path 532, PD3 543 for signal 533, and PD4 544for signal path 534, to obtain a PD current. Accordingly, a plot of thefour PD currents in the frequency domain is substantially the same asthose shown in FIG. 7. As shown, each PD current curve is a sine waveand each next wave is 90 degrees delay in phase. The quadrature pointsare intersection points among the four sub-spectrums and are againtunable to match with the four target frequencies assigned respectivelyto the four lasers. Optionally, four frequencies corresponding to fournearest neighbored quadrature points are selected to be the four targetfrequencies. Referring to FIG. 7, the free-spectral-range FSR associatedwith the DLI 530 is configured to be 4× of the channel spacing betweentwo neighboring target frequencies which can be just two neighboringchannels in a DWDM ITU grid. Optionally, the target frequencies at theITU grid have a channel spacing of 100 GHz, or 50 GHz, or 25 GHz, or12.5 GHz. Optionally, the target frequencies are selected from two ormore gridless channels. Optionally, any number of target frequencies canbe selected from corresponding number of quadrature points and used forlocking corresponding number of laser frequencies associated with thesilicon photonics transmission system.

Optionally, the wavelength locker 500 includes a resistive heater 5315disposed around the first arm 5311 of the DLI 530 for tuning the centerwavelength and group index to ensure that the quadrature points alignwith the corresponding target frequencies against possible temperaturedrift. Once the laser frequency is off the assigned target frequency dueto any reason such an environmental change or device aging, thewavelength locker is able to detect an error signal based on adifferential value obtained from the PD currents measured by thosephotodiodes of 541-544 in the four signal paths of 531-534. The errorsignal can be used as a feedback signal sent to the laser diode in thesilicon photonics transmission system 10 for retuning the laserfrequency back to the corresponding target frequency. Optionally, eachoutput signal path, including the first path 521 for providing a basesignal for measuring a reference intensity and each signal path 531-534for outputting an interference sub-spectrum, can be inserted a VOA tobalance the power in each path to overcome any possible powerfluctuation to ensure the error signal can be detected more accuratelywhich in turn ensure the wavelengths can be locked to the targetchannels more accurately. The first path 521 is inserted a VOA0 560.Each of the signal paths 531-534 is inserted a VOA, such as VOS1 561 forsignal path 531, VOA2 562 for signa path 532, VOA3 563 for signa path533, and VOA4 564 for signa path 534.

In some embodiments, each of the wavelength lockers 200 of FIG. 2, 300of FIG. 3, 400 of FIG. 5, and 500 of FIG. 6 is substantially similar tothe wavelength locker 100 that is placed after light signal from thesilicon photonics transmission system 10 is modulated and/ormultiplexed. In some alternative embodiments, each of the wavelengthlockers 200 of FIG. 2, 300 of FIG. 3, 400 of FIG. 5, and 500 of FIG. 6is placed prior to the light signal being modulated and/or multiplexed.The advantage of post modulation and post optical multiplexing of aplurality of wavelengths is that only a single DLI is needed.

In some embodiments, the wavelength locker 100 of FIG. 1 or 200 of FIG.2 is advantageous in application of locking wavelengths of dual-carriertransmission system. Optionally, as two or more wavelengths/frequenciesare carried in the input signal 111. In order to distinguish differentfrequencies in the PD current and the error signal deduced from the PDcurrents from different signal paths out of the DLI 200, a distinguisheddither frequency can be added to each different signal frequency. Thesedither frequencies can be added during signal modulation. As a result,the error signal can carry an ID-information about particular signalfrequency that is off a corresponding target frequency. The feedback ofthis error signal via the control loop can be sent to correspondinglaser diode that generates the identified signal and allow the laserdiode to be tuned to provide corrected laser frequency.

In an alternative aspect, the wavelength locker disclosed throughout thespecification above includes a calibration port that is able to, inaddition to use internal light source from the lasers of the siliconphotonics transmission system to calibrate the wavelength locker incurrent configuration, use an external light source with pre-calibratedtarget frequencies to calibrate the wavelength locker. In an example,FIG. 9 is a simplified diagram of the wavelength locker of FIG. 2 with acalibration port according to an embodiment of the present invention. Asshown, an external light signal 901 with a know frequency to be set as atarget frequency is sent via the primary output path 102 in reverseddirection. This external light signal 901 is tapped down as acalibration signal by the cross tap port 112 of the power tap coupler110. Through a waveguide 902, the calibration signal is guided to thesplitter 220 as an input with half power as a reference and another halfpower for tuning the DLI 230 to properly set a corresponding quadraturepoint of the interference spectrum and output as two sub-spectrumsmeasured in terms of two corresponding PD currents at the know targetfrequency. The calibrated wavelength locker 200A is able to performwavelength locking function when receiving tapped transmission signalout of the silicon photonics transmission system 10 for accuratelylocking the corresponding laser frequency to the target frequency.

In an alternative embodiment, an external light signal 1001 with a knowfrequency to be set as a target frequency is sent via the primary outputpath 102 in reversed direction, as shown in FIG. 10, but this externallight signal 1001 is tapped down as a calibration signal by the crosstap port 112 of the power tap coupler 110 and guided via a waveguide1002 to the MMI coupler 2301A with full power for tuning the DLI 230A toproperly set a corresponding quadrature point of the interferencespectrum and output as two sub-spectrums measured in terms of twocorresponding PD currents at the know target frequency. The calibratedwavelength locker 200B is able to perform wavelength locking functionwhen receiving tapped transmission signal out of the silicon photonicstransmission system 10 for accurately locking the corresponding laserfrequency to the target frequency.

In another alternative aspect, the present disclosure provides a siliconphotonics transmission system integrating a wavelength locker with atransmitter on a single SOI substrate. The transmitter includes one ormore DFB lasers for generating an optical signal multiplexed with one ormore frequencies. The wavelength locker associated with the siliconphotonics transmission system is substantially similar to one selectedfrom the wavelength locker shown in any one of FIG. 1, FIG. 2, FIG. 3,FIG. 5, and FIG. 6. The wavelength locker is configured with awaveguide-based DLI utilizing a tapped transmission signal forgenerating an interference spectrum and outputting at least twosub-spectrums in respective at least two signal paths. The at least twosub-spectrums cross each other in frequency domain to yield a pluralityof quadrature points at which one or more target frequencies selectedfrom certain DWDM channels in ITU grid or gridless channels areassigned. Each signal path is terminated by a photodiode to detecting aphotocurrent representing optical power varying with frequency. An errorsignal based on a differential photocurrent measured by all photodiodesrespectively terminated at all signal paths is deduced as thefrequencies generated by the one or more DFB lasers vary respectivelyaround the one or more target frequencies set at correspondingquadrature points. The wavelength locker is sending the error signal asa feedback signal through a control loop to corresponding one or morelaser diodes associated with the transmitter for tuning their operationsfor locking the one or more laser frequencies to respective desired oneor more target frequencies selected from certain DWDM channels.

Optionally, the silicon photonics transmission system includes thewavelength locker disposed prior or after a signal modulation deviceand/or a multiplexer for combining two or more signals with differentfrequencies into one transmission signal. Optionally, the siliconphotonics transmission system includes the wavelength lockersubstantially similar to one selected from the wavelength locker shownin FIG. 9 and FIG. 10, including a calibration port that is able toreceive an external light signal with pre-calibrated target frequencyfor calibrating the DLI of the wavelength locker.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A wavelength locker integrated with a siliconphotonics transmission system comprising: a silicon-on-insulator (SOI)substrate on which the silicon photonics transmission system is formed;a power tap coupler to receive a fraction of a transmission signal fromthe silicon photonics transmission system as an input signal with one ormore frequencies; a splitter configured to split the input signal to afirst signal in a first path and a second signal in a second path; afirst delay-line-interferometer (DLI) coupled to the second path toreceive the second signal and configured to generate an interferencespectrum and output a first sub-spectrum and a second sub-spectrumshifted in frequency domain by half of a free-spectral-range to form aplurality of quadrature points at respective one or more targetfrequencies; a first detector coupled to the first path for detecting areference power of the first signal; and a second detector and a thirddetector for respectively detecting a first power of the firstsub-spectrum and a second power of the second sub-spectrum depended onfrequency to generate an error signal fed back to the silicon photonicstransmission system for locking the one or more frequencies atcorresponding one or more target frequencies.
 2. The wavelength lockerof claim 1, wherein the power tap coupler is a waveguide device formedon the SOI substrate on which the silicon photonics transmission systemis formed, wherein the transmission signal with the one or morefrequencies is generated from respective one or more DFB lasersassociated with the silicon photonics transmission system.
 3. Thewavelength locker of claim 1, wherein the splitter comprises a waveguidedevice formed on the SOI substrate and configured as a 1×2 powersplitter so that the first signal and the second signal aresubstantially equal in power and each carries the at least twofrequencies.
 4. The wavelength locker of claim 1, further comprising asecond DLI combined with the first DLI to commonly couple to the secondpath via a 1×4 multimode interference (MMI) coupler to receive thesecond signal, the first DLI being configured to split half power of thesecond signal into two waveguide arms configured with a first arm beinglonger than a second arm by a first predetermined length to generate afirst interference spectrum with a first free-spectral-range equal totwice of a first channel spacing of a first set of channels, the secondDLI being configured to split remaining half power of the second signalinto two waveguide arms configured with a third arm being shorter than afourth arm by a second predetermined length to generate a secondinterference spectrum with a second free-spectral-range equal to twiceof a second channel spacing of a second set of channels, the second setof channels being interleaved with the first set of channels infrequency domain and the second channel spacing being equal to the firstchannel spacing.
 5. The wavelength locker of claim 1, wherein the firstdetector comprises a first photodiode to detect a reference photocurrentfor measuring the reference power of the first signal at the first path,and the second detector and the third detector respectively comprise asecond photodiode and a third photodiode to detect a first signalphotocurrent for measuring the first power of the first sub-spectrum andto detect a second signal photocurrent for measuring the second power ofthe second sub-spectrum, wherein each of the first signal photocurrentand the second signal photocurrent is normalized by the referencephotocurrent to reduce power fluctuation among the signal photocurrents.6. The wavelength locker of claim 1, further comprises a temperaturesensor disposed on the SOI substrate for monitoring local temperaturenear the first DLI for guiding a tuning of the interference spectrum andcalibrating the first DLI against thermal drift.
 7. The wavelengthlocker of claim 1, wherein the power tap coupler is configured toreceive a calibration signal sent reversely from the primary output pathincluding one or more frequencies set to respective target frequencies,and configured to deliver a fraction of the calibration signal via awaveguide to the splitter into the first DLI for calibrating the firstDLI.
 8. The wavelength locker of claim 1, wherein the power tap coupleris configured to receive a calibration signal sent reversely from theprimary output path including one or more frequencies set to respectivetarget frequencies, and configured to deliver a fraction of thecalibration signal via a waveguide directly to a 2×2 MMI coupler intothe first DLI for calibrating the first DLI.
 9. A silicon photonicstransmission system comprises one or more DFB lasers to output atransmission signal mixed with one or more frequencies to an outputport, a power tap coupler coupled to the output port to provide aprimary output path for outputting the transmission signal and asecondary path for providing a fraction of the transmission signal as aninput signal to a wavelength locker of claim 1 for locking correspondingone or more laser frequencies of the one or more DFB lasers torespective one or more target frequencies.
 10. The wavelength locker ofclaim 2, wherein the one or more target frequencies comprise one or moreDWDM channels at ITU grid or one of multiple gridless channels having achannel spacing selected from 100 GHz, 50 GHz, 25 GHz, and 12.5 GHz. 11.The wavelength locker of claim 4, wherein the first DLI comprises afirst 2×2 MMI coupler to output first two sub-spectrums of the firstinterference spectrum respectively to first two signal pathssubstantially in same power with a shifted phase of π equal to half ofthe first free-spectral range and a first resistive heater disposedaround the first arm for tuning the first interference spectrum to keepa first set of quadrature points respectively at correspondingfrequencies of the first set of channels, and the second DLI comprises asecond 2×2 MMI coupler to output second two sub-spectrums of the secondinterference spectrum respectively to second two signal pathssubstantially in same power with a shifted phase of 7C equal to half ofthe second free-spectral range and a second resistive heater disposedaround the fourth arm for tuning the second interference spectrum tokeep a second set of quadrature points respectively at correspondingfrequencies of the second set of channels.
 12. The wavelength locker ofclaim 5, wherein the first signal photocurrent and the second signalphotocurrent vary with frequency having opposite sign in frequencydifferential at each quadrature point.
 13. The wavelength locker ofclaim 5, further comprises one variable optical attenuator (VOA)disposed in the first path and each of the two signal paths forbalancing each of the first signal photocurrent and the second signalphotocurrent.
 14. The silicon photonics transmission system of claim 9,wherein the one or more target frequencies comprise DWDM channels of ITUgrid or gridless channels having a channel spacing selected from 100GHz, 50 GHz, 25 GHz, and 12.5 GHz.
 15. The silicon photonicstransmission system of claim 9, further comprises a modulator disposedprior to the output port before coupled to the power tap coupler. 16.The silicon photonics transmission system of claim 9, further comprisesa multiplexer prior to the output port before coupled to the power tapcoupler.
 17. The wavelength locker of claim 10, wherein the error signalis a ratio of a difference over a sum of the first power and the secondpower, the sum is normalized against the reference power, and each ofthe one or more frequencies is locked to a target frequency by tuningthe DFB lasers based on a change of sign of the error signal.
 18. Thewavelength locker of claim 10, wherein the first DLI comprises awaveguide device having two arms formed on the SOI substrate, an 1×2multimode interference (MMI) coupler to split the second signal into thetwo arms configured with a first arm being longer than a second arm by apredetermined length to generate the interference spectrum with afree-spectral-range equal to twice of the channel spacing.
 19. Thewavelength locker of claim 11, wherein the first set of channelscomprises DWDM channels at ITU grid or multiple gridless channels havinga channel spacing of 100 GHz or 50 GHz, the second set of channelscomprises DWDM channels at ITU grid or multiple gridless channels havinga channel spacing of 100 GHz or 50 GHz, the second set of channels areshifted from corresponding first set of channels by 50 GHz or 25 GHz.20. The wavelength locker of claim 18, wherein the first DLI furthercomprises a 2×2 MMI coupler to output the first sub-spectrum and thesecond sub-spectrum respectively to two signal paths substantially insame power with a relative phase shift equal to half of thefree-spectral range, and a resistive heater disposed around the firstarm for tuning the interference spectrum in frequency domain to keep thequadrature points of the first sub-spectrum crossing the secondsub-spectrum respectively at the one or more target frequencies.
 21. Thewavelength locker of claim 18, wherein the first DLI alternativelycomprises a 2×4 MMI coupler to output four sub-spectrums respectively tofour signal paths substantially in same power with a relative π/2 phaseshift equal to one-fourth of the free-spectral-range, and a resistiveheater disposed around the first arm for tuning the interferencespectrum in frequency domain to keep the quadrature points of the foursub-spectrums crossing each other respectively at one or more targetfrequencies.
 22. The wavelength locker of claim 18, wherein the firstDLI alternatively comprises a 2×N MMI coupler, wherein N is 2^(n) and nis natural number, to output N sub-spectrums respectively to N signalpaths in same power with a relative 2π/N phase shift equal to 1/N of thefree-spectral-range, and a resistive heater disposed around the firstarm for tuning the interference spectrum in frequency domain to keep thequadrature points of the N sub-spectrums crossing each otherrespectively at one or more target frequencies.